The present invention relates generally to frequency correlators and more particularly to such frequency correlators utilizing digital subtraction techniques.
In satellite tracking stations, sweep voltages to the receivers must be provided with the proper amplitude and duration to ensure that the receivers search in the correct doppler frequency range at the proper rate. The sweep voltages generated in a satellite tracking station are functions of the received doppler frequency signals. Thus, if the doppler, frequency signal is lost, the sweep voltage circuit must receive an indication of this so that it can hold a count of what the doppler frequency was when the signal was lost, so the sweep voltage will cause the receiver to start searching in the correct doppler frequency range. The present invention solves this critical problem by providing a logic 1 to the sweep voltage circuit when the received doppler frequency signals are coherent, i.e., within a predetermined frequency range, and a logic 0 when the received doppler frequency signals are incoherent, i.e., when the signal is lost.